Added handling for exponential expression.  Included splitting the
isTwoTermExpression() method into two separate methods, one for simple
and one for complex two term expression, because an exponential
expression could include a function.
Changed evaluator to use compute method, instead of using repeating code
to pick apart the expression and compute the arithmetic operations
Merge branch 'master' of https://github.com/mcmathe1/string-input-calculator.git